A hundred million years from now, when we're all dead and gone, a team of geologists will be digging in a field somewhere ...

... and they will discover, buried in the rocks below, a thin layer of sediment — very thin, about the width of a cigarette paper, says British stratigrapher Jan Zalasiewicz. That skinny strip, when they look close, will send what's called a "biostratigraphic signal" that something enormous happened back in our era, something life-changing, planet-reorganizing, even Earth-shaping. The evidence, when they look closely, will be visible in that same skinny layer all over the world. In her new book, The Sixth Extinction, Elizabeth Kolbert describes what they'll find.

During this same time, they will discover that animals and plants that used to be in one place — gingko trees in China, tulips in Asia, starlings in Europe — suddenly moved all over the world. Grasses found on one continent now strangely appear on four continents. Flowering plants, rats, goats, pigeons, kudzu, ants, inexplicably spread their territories across enormous oceans, climates, time zones. Specific life forms — chickens, cattle, roses, wheat, rice — turn up everywhere. Something moved them, though they may not know who or how. ...

Also at this time, bits of air trapped in the rock will show that the concentration of carbon dioxide in the atmosphere jumped sharply — to about 500 parts per million, higher than at any other point in the previous 800,000 years. So in this era, the chemical composition of the atmosphere changed, and changed very suddenly. ...

And down in the soil where supplies of nitrogen had been relatively rare, coming from existing populations of plants and animals — something changed, too. Out of nowhere, tons and tons of extra nitrogen appear. The supply jumps feverishly — feeding plants as never before. What happened?

Digging on six continents, geologists will discover that almost all of the Earth's major rivers, instead of winding and meandering across the planet's surface, were altered — blocked, re-routed or straightened. In some cases those rivers were dammed and pointed to new destinations. Enormous lakes were starved of water, and disappeared.

And surveying the continents, they will discover that roughly half the land on Earth that's free of ice had been significantly altered (yes, half) to provide space for crops, reservoirs, mining, logging, quarrying, housing, commerce or transportation. Wild spaces continued to exist, of course, mostly as rain forests, deserts, tundra and the higher mountain ranges, but they were a smaller and smaller proportion of the planet, sometimes crisscrossed by pipelines and affected by climate change. The densely built spaces, meanwhile ...

Millions and millions of years from now, all these changes will still be visible — to a geologist's practiced eye — right there in the rock, in that sliver. And the scale of the change and its subsequent effects will be so pronounced that geologists will want to give it a name, to mark the shift. Geologists do that when the change is big enough; we call the era of great dinosaurs the Jurassic; after that, the Cretaceous. What about this period? Right now, geologists call our time the Holocene, from the Greek for "entirely recent." Fair enough.

When Future Geologists Wonder ...

But looking backward, future geologists will want to know what caused all this change.

Volcanoes don't explain it. Incoming asteroids don't explain it. Climate cycles don't explain all of it — not the rivers, not the nitrogen, not all those structures and byways.

It will gradually become clear that some animal on the planet, proliferating, spreading, carrying, moving, building, designing, inventing, was largely responsible.

Which is why the chemist Paul Crutzen, sitting at a science meeting a few years ago, interrupted a talk where the speaker kept saying "Holocene" and blurted out, "Let's stop it!" The room got quiet. Crutzen had won a Nobel Prize for his work on the ozone layer, and laureates, I suppose, are allowed to have hissy fits — at least in science meetings.

Earth, Crutzen argued, is being dramatically changed, and the changer, this time, is us: humankind ("anthro" in the Greek). "We are no longer in the Holocene," he told the group. "We are in the Anthropocene." It's a coinage he may have borrowed from biologist Eugene Stoermer, but here's the logic: The Earth is no longer being shaped mainly by natural forces, forces that operate on their own with a logic of their own. Our little blue dot is now, increasingly, sculpted by one of its inhabitants. This is our planet now. We've taken over.

Some geologists think this idea is too radical, premature — not to mention vain, self-regarding, transient and overdramatized. They argue that we humans have made little scratches on the surface of things, but have changed nothing fundamental. The world doesn't need us. It will hardly notice when we're gone. All we will leave is a layer in the sand.

But the idea isn't going away. Jan Zalasiewicz heads the committee that is considering new names for our present epoch, and he supports a change. There are competing candidates — "Anthrocene" (from reporter Andy Revkin), the "Homogenocene" (from biologist Michael Samways). The International Commission on Stratigraphy will hold a meeting in 2016 to consider candidates for renaming this epoch we're in. There will probably be a vote, and the question for the group will be, is planet Earth under new management — or no?